In order to control the temperature of the water in a water tank of a water heater, one or more temperature probes are usually permanently mounted in the wall of the tank and project inside the tank in contact with the water therein. In recent years, and due to the increase cost of electricity, more sophisticated control systems have been developed to control the operation of the water heater whereby to operate the resistive heating elements during non-peak hours when electricity costs are lowest to create a reserve of hot water for use during peak-hours to reduce the operating time of the elements when electricity is at a premium.
Utilities also have a need to take control of water heaters to prevent overcharging its grid during peak hours and to use its electricity when the demand is low as well as the cost. Such control would create an equilibrium or a mean demand of electricity of the grid. Still further, after power outages when electricity is restored, it is desirable for the utility to control the load to prevent an overload on the grid. Control devices have been developed and continue to be developed to remotely control the actuation of the resistive heating elements of the water heaters at the customer locations while attempting to satisfy the hot water demands of the customers. In order to achieve this, it is necessary to determine the hot water reserve inside the tank of the water heater. Accordingly, temperature sensors are required to provide actual temperature values at distinct locations of the water tank and particularly in the uppermost region of the tank where the hottest water is located and drawn.
Thermistors are excellent electrical components for sensing temperature variations as its resistance varies with temperature changes. Because the water tank is constructed of steel material which is a good thermal conductor, by sensing the temperature of the steel by mounting thermistors there against, the temperature of the water inside the tank can be fairly accurately determined. To ensure a desired hot water volume in the tank, it is necessary to monitor the temperature of the water in the region of the heating elements. Because the thermistors need to be mounted outside the water tank wall, there is often no access to these sensor components once the thermal insulation foam is injected about the tank wall sealing access to these devices and their wiring. Water heaters are now being manufactured with three resistive heating elements to achieve better control of the water temperature in the tank and by selecting the operation of the various heating elements, some of which may be of a much lower wattage than the others. Thus, three thermistors would be required to monitor the temperature of the tank wall in the region of the three resistive heating elements. Because of the insulation blown inside the gap formed between the outer skin of the water heater and the water tank, there would be a need to form foam dams about the thermistors and access doors in the outer casing for access to the thermistors in case of a defect. This is not practical and adds to cost. Also, it complicates the troubleshooting and repair and does not remedy a defective wiring problems that could occur between the thermistors.
Mounting the thermistors on the tank wall and sealing them with the foam insulation would require replacing the hot water heater to remedy a defective thermistor problem or a defective associated wiring problem inside the water heater.